How can I tell whether the soil is too acid or not acid enough?
Plants will not respond properly to soil that does not suit them. To be sure, send a sample of soil to your nearest agricultural experiment station for a free analysis. In fact, if you are mixing potting soil in sizable quantities and are uncertain about its acidity or alkalinity (pH), it is always a good idea to have a sample analyzed. A soil with a pH of 7 is said to be neutral; below that, it is acid; above that, alkaline. The pH scale ranges from 4 to 9. Most plants do well with a pH of 6 to 7.
Alternatively, you can purchase a pH meter. They are inexpensive, and most are accurate enough to give you good results.
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